This invention relates generally to deformable mirrors and, more particularly, mirrors of which the surface can be rapidly changed in contour. The deformable mirror is a key component in adaptive optics systems for various applications. For laser directed energy weapons, adaptive optics systems may be used to compensate for optical wavefront distortions caused by transmission of a laser beam through the atmosphere. Similarly, many large aperture telescopes employ adaptive optics to compensate for atmospheric turbulence. There are also a number of commercial products that employ adaptive optics to map imperfections in the human eye and provide a tool for adjusting vision correction procedures.
All of these applications have in common the need to control the surface contour of a mirror in real time, i.e., rapidly enough to compensate for changes as they occur. A deformable mirror uses a matrix of individually controlled actuators, usually mounted behind the mirror surface. In one common type, the actuators are bi-directional pistons, each of which moves an assigned segment or region of the mirror. The mirror surface may extend continuously across the actuators or may consist of independently movable mirror segments with small gaps between the segments. In the latter type, movement of any one actuator has no influence on any other segments. Although this arrangement simplifies control of the actuators, the gaps between mirror segments are unacceptable for most applications that use laser beams. In the former type, in which a continuous mirror surface is controlled by independent bi-directional actuators, movement of any one actuator will also influence neighboring surface regions to some degree. An overwhelming drawback to the use of independently controlled bi-directional actuators is the relatively high cost of manufacture and assembly of the actuators.
A less costly alternative is to use a thin membrane to support the deformable mirror surface and to employ electrostatic actuators beneath the mirror surface, rather than electromechanical or piezoelectric actuators. In addition to having a relatively low cost, the thin membrane mirror surface has no discontinuities and, therefore, minimizes optical distortions between adjacent mirror regions controlled by separate actuators. Because of its advantages over more expensive mirrors, the thin membrane mirror is widely used in many adaptive optics applications.
Unfortunately, however, the continuity of the thin membrane mirror gives rise to a significant, and sometimes critical, disadvantage. Each actuator affects not only the membrane region immediately above the actuator but, to a lesser degree, it also affects every other region of the membrane as well. In other words, in addition to the desired local deformations produced by selective operation of one or more of the actuators, the entire membrane is deformed to a significant degree in a downward direction. (For convenience of discussion, it is assumed that the deformable mirror is oriented horizontally, with actuators mounted beneath the mirror surface. It will be understood, of course, that the mirror may have any desired orientation.)
The downward deformation of the entire membrane results in a concave and generally parabolic bias to the membrane. In many applications, the optical focus resulting from this concave bias may be removed by adding a corrective lens. This is usually not an acceptable solution, however, because the degree of the concave bias will vary with the current need for deforming the mirror to effect wavefront corrections. For a wavefront that requires no correction at a certain instant in time, no actuators are in operation and the membrane remains perfectly flat. Obviously, at this time no concave bias exists in the membrane and no compensation for concave bias is needed. As soon as one or more actuators pull down on the membrane to produce a desired distortion of the mirror, the concave bias of the mirror must be taken into account. One way to do this is to compensate for the bias with an adjustable rather than a fixed lens, but this obviously adds another layer of complexity to the mirror.
An example of the prior art deformable mirror system illustrated in FIGS. 1–3 is taught in some detail by U.S. Pat. No. 6,108,121, entitled “Micromachined High Reflectance Deformable Mirror.”
It will be appreciated, therefore, that there is a need for a deformable mirror of the thin membrane type that can be deformed not only to compensate for wavefront distortions in an incident light beam, but also to compensate for the concave bias that is inherent in thin membrane deformable mirrors. The present invention accomplishes this goal.